The goal of these experiments are to image the magnetic field associated with the action currents in planar activation wave fronts using a combination of SQUID microscope imaging and epifluorescent imaging. During the course of these experiments, advanced instrumentation will be developed to allow us to better achieve the above goals; specifically, we will combine the imaging modalities for simultaneous electrical and magnetic measurements. This experiment is extremely important, in that the preliminary data we have obtained seems to support the prediction of the doubly-anisotropic cardiac bidomain model that there are action currents flowing along a planar wave front, in contrast to the expectation from older monodomain or equal-anisotropy bidomain models for which the currents are perpendicular to the wave front. The results of these experiments will provide valuable insights into the role of the bidomain model describing cardiac wave propagation, and the information content of the ECG and MCG, and thus, contribute to advanced modeling of cardiac tissue in three-dimensions.